It's advisable to do so. Using the proper abbreviation gives a clear signal to clients, customers, vendors, and other entities dealing with your business that it is an entity. If it is not clear that the business is a limited liability company or a corporation, in the event of a dispute, a court might find that the owners entered into a transaction in their individual capacities. This could expose you to personal liability. Standard business documents, such as letters, invoices, contracts, and business cards, should contain the full business entity's name, including the proper abbreviation. You may have a little more leeway with promotional items like pens or water bottles, but you should discuss any such exceptions with your attorney.